1. Field of the Invention
The present invention relates to an anode having an anode active material layer on an anode current collector and a battery including the same.
2. Description of the Related Art
In recent years, portable electronic devices such as combination cameras (videotape recorder), mobile phones, and notebook personal computers have been widely used, and it is strongly demanded to reduce their size and weight and to achieve their long life. Accordingly, as a power source, a battery, in particular a light-weight secondary batter capable of providing a high energy density has been developed.
Specially, a secondary battery using insertion and extraction of lithium for charge and discharge reaction (so-called lithium ion secondary battery) is extremely prospective, since such a lithium ion secondary battery provides a higher energy density compared to a lead battery and a nickel cadmium battery. The lithium ion secondary battery includes a cathode and an anode that are opposed to each other with a separator in between and an electrolytic solution impregnated in the separator. The anode has an anode active material layer on an anode current collector.
As an anode active material contained in the anode active material layer, a carbon material such as graphite has been widely used. However, in recent years, as the high performance and the multi functions of the portable electronic devices are developed, improving the battery capacity is further demanded. Thus, it has been considered to use silicon or the like instead of the carbon material. Since the theoretical capacity of silicon (4199 mAh/g) is significantly higher than the theoretical capacity of graphite (372 mAh/g), it is prospected that the battery capacity is thereby significantly improved.
The lithium ion secondary battery provides a high energy density. Meanwhile, the lithium ion secondary battery has some disadvantages that should be solved. Firstly, when charged, the anode active material inserting lithium is highly active. Thus, the electrolytic solution tends to be easily decomposed and lithium tends to be inactivated. Secondly, when the separator is broken by strong external force, internal short circuit may be generated. Thirdly, when the anode active material is swollen and shrunk during charge and discharge, the anode active material layer may be separated. All the foregoing disadvantages may become a cause to lower the cycle characteristics and the safety.
To solve the various disadvantages of the lithium ion secondary battery, various techniques have been considered. Specifically, to improve the cycle characteristics and the load characteristics, a technique of coating the surface of carbon particles as an anode active material with a hydrate of aluminum oxide by dipping method has been proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 2003-257419). Further, to prevent abnormal overheat due to internal short circuit, a technique of forming a porous insulating layer such as alumina on the surface of the anode active material layer by vapor-phase deposition method such as sputtering method has been proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 2005-183179). Furthermore, to prevent internal short circuit due to overheat, a technique of forming a porous heat-resistant layer containing an insulative filler such as alumina and a binder between the separator and the anode active material layer by coating method has been proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 2006-120604). Moreover, to prevent internal short circuit, a technique of forming a porous protective film composed of solid particles such as alumina powder and a resin binder on the surface of the anode active material layer by coating method has been proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 07-220759). Moreover, to improve the capacity characteristics and the cycle characteristics, a technique of covering the surface of the anode active material with an inorganic ion conductive film composed of lithium-aluminum hydroxide compound by dipping method has been proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 09-171813). Moreover, to improve the cycle characteristics, a technique of covering a partial surface of the anode active material with a metal oxide by liquid-phase deposition method has been proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 2007-141666).